Electromagnetic waves in the milli, submilli or light wave region are conveyed through transmission lines in a dielectric mode, a surface wave mode, a wave-guide mode or in any combination thereof. Such transmission lines utilize dielectric materials either partially or entirely as the medium for conveying the electromagnetic waves described above.
Conventional dielectric materials for conveying the electromagnetic waves include polyethylene, polypropylene and polytetrafluoroethylene. Examples of materials having low dielectric constants useful for surrounding these materials included foamed plastics including polyethylene and polypropylene. Such materials have numerous discrete bubbles created by the foaming agent. A problem with these foamed plastics is that some of the foaming agent remains causing disadvantages, such as increased dielectric loss, difficulty in controlling dielectric constant differences in degree of foaming especially at boundary areas, difficulty in forming bubbles with diameters less that several fractions (1/5-1/6 .gamma.) of the wave length to be transmitted and difficulty in fabrication of the cable. These foamed materials have not, therefore, found uses as materials for surrounding or jacketing dielectrics.